This invention partially relates to an improvement on our prior patents, U.S. Pat. No. 6,099,605 and its division, U.S. Pat. No. 6,432,855; the first issued Aug. 8, 2000 and the second Aug. 13, 2002. Those patents relate to a ceramic material which is an orthorhombic boride of the general formula: AlMgB14. Crystallographic studies indicate that the metal sites are not fully occupied in the lattice so that the true chemical formula may be closer to Al0.75Mg0.78B14 which is contemplated by the formula here used as AlMgB14. The ceramic is a superabrasive, and in most instances provides a hardness of 30 GPa or greater. This invention relates to an improvement, involving the use of a binder phase to modify properties of AlMgB14 and other hard materials for certain uses such as machine tools.
Advanced machining tools must possess both good hardness and reasonable fracture toughness, where hardness is defined as resistance to plastic indentation and toughness is a measure of a material's ability to absorb an impact without catastrophic fracture. Tungsten carbide (WC) for example is moderately hard but quite brittle; addition of cobalt as a binder phase enables monolithic tools fashioned from this material to better tolerate impacts such as those encountered during discontinuous cutting that would otherwise result in fracture and loss of the tool. The WC/Co composite is therefore characterized as a hard and brittle material dispersed in a continuous ductile matrix. The present invention involves discovery of a binder phase for AlMgB14 and other hard materials.
Recent efforts to develop the ultra-hard AlMgB14 into a next-generation cutting tool have motivated studies into the fracture resistance of this material and in possible binder phase additions. For a binder to be compatible, it must exist as a liquid phase within a temperature range that avoids undesirable decomposition of the active material, while also possessing a similar (or lower) surface energy to enable good “wetting” of each grain. Furthermore, the binder must possess sufficient ductility to absorb and dissipate the energy associated with an advancing crack tip, while retaining adequate strength to prevent failure under typical tensile, torsional, or shear loading. Several requirements exist for liquid phase sintering. First, the temperature must be sufficiently high so that the binder phase becomes completely liquid. A favorable contact angle must exist between the liquid binder phase and the solid base material. In other words, the relative surface energies of the two phases must be sufficiently low so that the liquid “wets” or completely covers each hard particle. Moreover, an appropriate volume fraction of binder phase must be present. In the case of insufficient quantity of binder, the tool may contain excessive porosity and lack mechanical strength. In the case of excessive amounts of binder phase, the mechanical properties of the tool will be determined primarily by the binder itself rather than that of the harder base material. In addition, excessive binder can result in liquid phase “squeeze-out” during sintering and undesired shape changes.
A consolidation temperature of 1400° C., as applied to the AlMgB14 materials, precludes use of conventional binder metals such as nickel and cobalt, which melt at temperatures of 1453° C. and 1495° C., respectively. Consequently, an alternative binder metal was sought with a constraint that its freezing range should lie between 1380° C. and 1400° C.
It is therefore a primary object of the present invention to develop a suitable binder phase for use with ultra-hard AlMgB14 and other hard materials.
Another object of the present invention is to develop a binder phase which “wets” or completely covers each hard particle of AlMgB14 or other hard materials.
Yet another object of the present invention is to provide a binder phase for AlMgB14 and other hard materials which can be used in appropriate quantities to tailor good hardness and reasonable fracture toughness for AlMgB14 and other hard materials so that they can be used suitably in industrial machining and grinding applications.
The method and means of accomplishing these and other objectives of the invention will become apparent from the written description given below.